DesiPilot:
You are technically correct that less fuel is burned to climb to 1500 feet at TOGA vs. FLEX, but the amount is negligible. The wear and tear on the engines of a TOGA thrust creates considerable engine maintenance cost because the engine ends up coming off for overhaul sooner.
However, the NET fuel burn is less with FLEX. With FLEX since engine wear is less the incremental fuel burn for engine wear is less i.e. Fuel Burn Correction factor. That affects each flight for the entire flight and results in significant savings.
Sometimes we need to stand back and look at the total net fuel impact and not isolate analysis by phase of flight.